This invention relates to the production of paper or paper board by a process comprising forming an aqueous cellulosic suspension, adding a polymeric retention aid to the suspension to form flocs, degrading the flocs by shearing the suspension to form microflocs, aggregating the microflocs by adding to the suspension an aqueous composition of an anionic bridging coagulant, draining the aggregated suspension to form a sheet, and drying the sheet. Processes of this general type are well known. For instance the Hydrocol (trade mark) process involves these process steps and utilises bentonite (i.e. an anionic swelling clay) as the anionic bridging coagulant. Such processes are described in, for instance, U.S. Pat. Nos. 4,753,710, 4,913,775 and EP-A-707673.
The formation of the flocculated suspension generally involves the addition of one or more cationic polymers to the suspension. For instance the polymeric retention aid is often a high molecular weight cationic polymer, and/or other cationic polymers may be added at earlier stages in the process. For instance cationic starch or other strength additive can be added to increase strength and/or low molecular weight cationic polymers can be added to improve retention and/or for other purposes, such as controlling pitch in the thick stock.
In order to improve the visual appearance of the dried sheets, it is conventional to add an anionic material which will alter the visual appearance of the sheet, such as a pigment or dye or, usually, an optical brightening agent. For reasons of convenience and thorough mixing, these anionic materials are always added at a relatively early stage in the process, certainly before the retention aid and often even at the thick stock stage, for instance in the mixing chest.
Thus a typical process comprises adding an anionic optical brightener to the thick stock with or prior to any filler that is required and then adding cationic starch and/or low molecular weight cationic coagulant (which may have also been added to the thick stock as a pitch control additive), then adding the cationic or other polymeric retention aid and then the anionic bridging coagulant.
Processes of this type have been operated on a very large scale for many years.
In all paper making processes it is desirable to obtain optimum performance utilising a minimum amount of chemical additives. Thus the mill operator wants to achieve optimum pitch control, strength, retention and drainage or other dewatering using a minimum amount of polymer, and optimum visual appearance using a minimum amount of optical brightener, dye or pigment.